Running a perl program

Explicitly invoking the Perl compiler/interpreter
(note that >>> is prompt only)
>>>perl program.pl
Many command line options are available here eg.
-w print all warnings
-c check but do not execute
-d run under debugger control

Implicitly invoking the Perl compiler/interpreter
It is system specific. e.g. Unix
#!/usr/bin/perl
print "Please enter your name: ";
 This is popular shebang notation. Also you can do following:
#!/usr/bin/perl -w
for warning check etc like we do with command line options.

More operators in perl

x         The repetition operator. Returns a string consisting of the left operand repeated the number of times specified by the right operand. In an array context, if the left operand is a list in parens, it repeats the list.

print '-' x 80; # print row of dashes
print '-' x80; # illegal, x80 is identifier
print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
@ones = (1) x 80; # an array of 80 1's
@ones = (5) x @ones; # set all elements to 5

x=        The repetition assignment operator. Only works on scalars.

..          The range operator, which is really two different operators depending on the context. In an array context, returns an array of values counting (by ones) from the left value to the right value. This is useful for writing "for (1..10)" loops and for doing slice operations on arrays.

In a scalar context, .. returns a boolean value. The operator is bistable, like a flip-flop, and emulates the line-range (comma) operator of sed, awk, and various editors. Each .. operator maintains its own boolean state. It is false as long as its left operand is false. Once the left operand is true, the range operator stays true until the right operand is true, AFTER which the range operator becomes false again. (It doesn't become false till the next time the range operator is evaluated. It can test the right operand and become false on the same evaluation it became true (as in awk), but it still returns true once. If you don't want it to test the right operand till the next evaluation (as in sed), use three dots (...) instead of two.) The right operand is not evaluated while the operator is in the "false" state, and the left operand is not evaluated while the operator is in the "true" state. The precedence is a little lower than

and &&. The value returned is either the null string for false, or a sequence number (beginning with 1) for true. The sequence number is reset for each range encountered. The final sequence number in a range has the string 'E0' appended to it, which doesn't affect its numeric value, but gives you something to search for if you want to exclude the endpoint. You can exclude the beginning point by waiting for the sequence number to be greater than 1. If either operand of scalar .. is static, that operand is implicitly compared to the $. variable, the current line number.



Examples:



As a scalar operator:

if (101 .. 200) { print; } # print 2nd hundred lines



next line if (1 .. /^$/); # skip header lines



s/^/> / if (/^$/ .. eof()); # quote body



As an array operator:

for (101 .. 200) { print; } # print $_ 100 times



@foo = @foo[$[ .. $#foo]; # an expensive no-op

@foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items

More operators in perl

x         The repetition operator. Returns a string consisting of the left operand repeated the number of times specified by the right operand. In an array context, if the left operand is a list in parens, it repeats the list.

print '-' x 80; # print row of dashes
print '-' x80; # illegal, x80 is identifier
print "\t" x ($tab/8), ' ' x ($tab%8); # tab over
@ones = (1) x 80; # an array of 80 1's
@ones = (5) x @ones; # set all elements to 5

x=        The repetition assignment operator. Only works on scalars.

..          The range operator, which is really two different operators depending on the context. In an array context, returns an array of values counting (by ones) from the left value to the right value. This is useful for writing "for (1..10)" loops and for doing slice operations on arrays.

In a scalar context, .. returns a boolean value. The operator is bistable, like a flip-flop, and emulates the line-range (comma) operator of sed, awk, and various editors. Each .. operator maintains its own boolean state. It is false as long as its left operand is false. Once the left operand is true, the range operator stays true until the right operand is true, AFTER which the range operator becomes false again. (It doesn't become false till the next time the range operator is evaluated. It can test the right operand and become false on the same evaluation it became true (as in awk), but it still returns true once. If you don't want it to test the right operand till the next evaluation (as in sed), use three dots (...) instead of two.) The right operand is not evaluated while the operator is in the "false" state, and the left operand is not evaluated while the operator is in the "true" state. The precedence is a little lower than

and &&. The value returned is either the null string for false, or a sequence number (beginning with 1) for true. The sequence number is reset for each range encountered. The final sequence number in a range has the string 'E0' appended to it, which doesn't affect its numeric value, but gives you something to search for if you want to exclude the endpoint. You can exclude the beginning point by waiting for the sequence number to be greater than 1. If either operand of scalar .. is static, that operand is implicitly compared to the $. variable, the current line number.



Examples:



As a scalar operator:

if (101 .. 200) { print; } # print 2nd hundred lines



next line if (1 .. /^$/); # skip header lines



s/^/> / if (/^$/ .. eof()); # quote body



As an array operator:

for (101 .. 200) { print; } # print $_ 100 times



@foo = @foo[$[ .. $#foo]; # an expensive no-op

@foo = @foo[$#foo-4 .. $#foo]; # slice last 5 items

Comparison operators in perl

cmp --String comparison, returning -1, 0, or 1.
<=>    Numeric comparison, returning -1, 0, or 1. (looks like flying saucer)

 =~       Certain operations search or modify the string "$_" by default. This operator makes that kind of operation work on some other string. The right argument is a search pattern, substitution, or translation. The left argument is what is supposed to be searched, substituted, or translated instead of the default "$_". The return value indicates the success of the operation. (If the right argument is an expression other than a search pattern, substitution, or translation, it is interpreted as a search pattern at run time. This is less efficient than an explicit search, since the pattern must be compiled every time the expression is evaluated.) The precedence of this operator is lower than unary minus and autoincrement/decrement, but higher than everything else.


!~       Just like =~ except the return value is negated.

Operators in perl - bitwise operators

Bitwise operators
>> - Bitwise shift shifting the bits in the left operand to the right by the number of bits indicated by the right operand.


<< - Bitwise shift shifting the bits in the left operand to the left by the number of bits indicated by the right operand.

~ - Ones complement

Operators in perl - relational operators

Note eq is for string and == is for integers. So here are operator for int and corresponding operator for strings.

Comparison	Numeric	String
Equal	==	eq
Not equal	!=	ne
Greater than	>	gt
Less than	<	lt
Greater than or equal to	>=	ge
Less than or equal to	<=	le

Logical operators
 &&,and
||      or

Logical expression
The next few structures rely on a test being true or false. In Perl any non-zero number and non-empty string is counted as true. The number zero, zero by itself in a string, and the empty string are counted as false. Here are some tests on numbers and strings.

$a == $b                # Is $a numerically equal to $b?
                        # Beware: Don't use the = operator.
$a != $b                # Is $a numerically unequal to $b?
$a eq $b                # Is $a string-equal to $b?
$a ne $b                # Is $a string-unequal to $b?

You can also use logical and, or and not:

($a && $b)              # Is $a and $b true?
($a || $b)              # Is either $a or $b true?
!($a)                   # is $a false?

Note that perl is loosely typed language and allows you anything..which you do with
int etc to do with strings. So here is one eg.
So far we have been testing numbers, but there is more to life than numbers. There are strings too, and these need testing too.

$name  = 'Mark';

$goodguy = 'Tony';

if ($name == $goodguy) {
        print "Hello, Sir.\n";
} else {
        print "Begone, evil peon!\n";
}   

Something seems to have gone wrong here. Obviously Mark is different to Tony, so why does perl consider them equal?
Mark and Tony are equal -- numerically. We should be testing them as strings, not as numbers. To do this, simply substitute == for eq and everything will work as expected.

Understanding 0 in perl

There are many Perl functions which test for Truth. Some are if, while, unless . So it is important you know what truth is, as defined by Perl, not your tax forms. There are three main rules:

1. Any string is true except for "" and "0".
2. Any number is true except for 0. This includes negative numbers.
3. Any undefined variable is false. A undefined variable is one which doesn't have a value, ie has not been assigned to.

Some example code to illustrate the point:

&isit;                   # $test1 is at this moment undefined

$test1="hello";         # a string, not equal to "" or "0"
&isit;

$test1=0.0;             # $test1 is now a number, effectively 0
&isit;

$test1="0.0";           # $test1 is a string, but NOT effectively 0 !
&isit;

sub isit {
        if ($test1) {                           # tests $test1 for truth or not
                print "$test1 is true\n";
        } else {                                # else statement if it is not true
                print "$test1 is false\n";
        }
}

The first test fails because $test1 is undefined. This means it has not been created by 
assigning a value to it. So according to Rule 3 it is false. The last two tests are
interesting. Of course, 0.0 is the same as 0 in a numeric context. But it is not the
same as 0 in a string context, so in that case it is true.    

So here we are testing single variables. What's more useful is testing the result of an expression. For example, this is an expression; $x * 2 and so is this; $var1 + $var2 . It is the end result of these expressions that is evaluated for truth.
An example demonstrates the point:

$x=5;
$y=5;

if ($x - $y) {
        print '$x - $y is ',$x-$y," which is true\n";
} else {
        print '$x - $y is ',$x-$y," which is false\n";
}   

The test fails because 5-5 of course is 0, which is false. The print statement might look a little strange. Remember that print is a list operator? So we hand it a list. First item, a single-quoted string. It is single quoted because it we do not want to perform variable interpolation on it. Next item is an expression which is evaluated, and the result printed. Finally, a double-quoted string is used because we want to print a newline, and without the doublequotes the \n won't be interpolated.
What is probably more useful than testing a specific variable for truth is equality testing. For example, has your lucky number been drawn?

$lucky=15;
$drawnum=15;

if ($lucky == $drawnum) {
        print "Congratulations!\n";
} else {
        print "Guess who hasn't won!\n";
}   

The important point about the above code is the equality operator, == .

if else and unless in perl

Perl provides two forms of simple conditional statement. if-else
Of course Perl also allows if/then/else statements. These are of the following form:

if ($a)
{
        print "The string is not empty\n";
}
else
{
        print "The string is empty\n";
}

For this, remember that an empty string is considered to be false. It will also give an "empty" result if $a is the string 0.
Using ! and unless
Similarly we can have ! not operator.
In this, it is important to notice that the elsif statement really does have an "e" missing.
Sometimes, it is more readable to use unless instead of if (!...) . The switch-case statement familiar to C programmers are not available in Perl. You can simulate it in other ways. See the manual pages.

elsif 

Note that e is absent in else.

if ($age > $max) {
        print "Too old !\n";
} elsif ($age < $min) { 
        print "Too young !\n"; 
} else { 
        print "Just right !\n"; 
}

Escape character in perl

There's something else new in the code above. The \ . You can see what this does -- it 'escapes' the special meaning of $ .
Escaping means that just the $ symbol is printed instead of it referring to a variable.
Actually \ has a deeper meaning -- it escapes all of Perl's special characters, not just $ . Also, it turns some non-special characters into something special. Like what ? Like n . Add the magic \ and the humble 'n' becomes the mighty NewLine ! The \ character can also escape itself. So if you want to print a single \ try:

print "the MS-DOS path is c:\\scripts\\";   

Oh, '\' is also used for other things like references.

Datatypes in perl

Perl has 3 main datatypes - scalars, arrays and hashes which are prefixed with a symbol such as $ @ % when used as variables.

Scalars
So Perl is working, and you are working with Perl. Now for something more interesting than simple printing. Variables. Let's take simple scalar variables first. A scalar variable is a single value. Like $var=10 which sets the variables $var to the value of 10. Later, we'll look at lists like arrays and hashes, where @var refers to more than one value. For the moment, remember that Scalar is Singular.

Also note that these scalar value can be int , float strings what you have seen in some languages like c.
More of arrays and hashes see in other posts.

Loosely typed language
The idea of forcing a programmer to declare what sort of variable is being created is called typing. As Perl doesn't by default enforce any rules on typing, it is said to be a loosely typed language, as opposed to something like C++ which is strongly typed. One must keep in mind that perl was designed for shorter and fast things...rather than v.big projects.

Shebang

#!usr/bin/perl
#! is called shebang.

The function of the 'shebang' line is to tell the shell how to execute the file. Under UNIX, this makes sense. Under Win32, the system must already know how to execute the file before it is loaded so the line is not needed.

However, the line is not completely ignored, as it is searched for any switches you may have given Perl (for example -w to turn on warnings).

You may also choose to add the line so your scripts run directly on UNIX without modification, as UNIX boxes probably do need it. Win32 systems do not.

Perl Reserved Words

Reserved Literals    




 
__END__
  
indicates the logical end of the script (^D and ^Z are synonyms)
 
 
__FILE__
  
current filename
 
 
__LINE__
  
current line number
 

 
Reserved Filehandles
 





 
<>
  
Null filehanedle - input from either stdin, or files specified on the command line. Psuedonym for internal filehandle .
 
 
DATA
  
read data only from the main script, but not from any required file or evaluated string.
 
 
STDERR
  
Output to stderr
 
 
STDIN
  
Input from stdin
 
 
STDOUT
  
Output to stdout
 

 
Reserved Variables
 
















































 
$_
  
The default input and pattern-searching space. The following pairs are equivalent: 

    while (<>) {...}    while ($_ = <>) {...}   # only equivalent in while! 

    /^Subject:/         $_ =~ /^Subject:/ 

    tr/a-z/A-Z/         $_ =~ tr/a-z/A-Z/ 

    chop                chop($_) 

Also: $ARG
 
 
@_
  
The parameters passed to a subroutine. The array itself is local to the subroutine, but its values are references to the variables that are passed so updates to the members of the array will update the corresponding parameter value.
 
 
<digit>
  
Contains the subpattern from the corresponding set of parentheses in the last pattern matched, not counting patterns matched in nested blocks that have been exited already. These variables are all read-only.
 
 
$&
  
The string matched by the last successful pattern match (not counting any matches hidden within a BLOCK or eval() enclosed by the current BLOCK). This variable is read-only. 

Also: $MATCH
 
 
$`
  
The string preceding whatever was matched by the last successful pattern match, not counting any matches hidden within a BLOCK or eval enclosed by the current BLOCK. This variable is read-only. 

Also: $PREMATCH
 
 
$'
  
The string following whatever was matched by the last successful pattern match (not counting any matches hidden within a BLOCK or eval() enclosed by the current BLOCK). Example:  

     $_ = 'abcdefghi'; 

     /def/; 

     print "$`:$&:$'\n";         # prints abc:def:ghi 

This variable is read-only. 

Also: $POSTMATCH
 
 
$+
  
The last bracket matched by the last search pattern. This is useful if you don't know which of a set of alternative patterns matched. For example: 

     /Version: (.*)|Revision: (.*)/ && ($rev = $+); 

This variable is read-only. 

Also: $LAST_PAREN_MATCH
 
 
$*
  
Set to 1 to do multiline matching within a string, 0 to tell Perl that it can assume that strings contain a single line, for the purpose of optimizing pattern matches. Pattern matches on strings containing multiple newlines can produce confusing results when " $* " is 0. Default is 0. Note that this variable only influences the interpretation of " ^ " and " $ ". A literal newline can be searched for even when $* == 0 . The '/m' modifier should be used instead when pattern matching. 

Also: $MULTILINE_MATCHING
 
 
$.
  
The current input line number of the last filehandle that was read. This variable should be considered read-only. Remember that only an explicit close on the filehandle resets the line number. Since " <> " never does an explicit close, line numbers increase across ARGV files. 

Also: $NR, $INPUT_LINE_NUMBER, input_line_number HANDLE EXPR
 
 
$/
  
The input record separator, newline by default. Works like awk 's RS variable, including treating blank lines as delimiters if set to the null string. You may set it to a multicharacter string to match a multi-character delimiter. Note that setting it to "\n\n" means something slightly different than setting it to "" , if the file contains consecutive blank lines. Setting it to "" will treat two or more consecutive blank lines as a single blank line. Setting it to "\n\n" will blindly assume that the next input character belongs to the next paragraph, even if it's a newline.  

   undef $/; 

    $_ = ;          # whole file now here 

    s/\n[ \t]+/ /g; 

Also: $RS, $INPUT_RECORD_SEPARATOR, input_record_separator HANDLE EXPR
 
 
$|
  
If set to nonzero, forces a flush after every write or print on the currently selected output channel. Default is 0. Note that STDOUT will typically be line buffered if output is to the terminal and block buffered otherwise. Setting this variable is useful primarily when you are outputting to a pipe, such as when you are running a Perl script under rsh and want to see the output as it's happening. 

Also: $OUTPUT_AUTOFLUSH, autoflush HANDLE EXPR
 
 
$,
  
The output field separator for the print operator. Ordinarily the print operator simply prints out the comma separated fields you specify. In order to get behavior more like awk, set this variable as you would set awk 's OFS variable to specify what is printed between fields. 

Also: $OFS, $OUTPUT_FIELD_SEPARATOR, output_field_separator HANDLE EXPR
 
 
$\
  
The output record separator for the print operator. Ordinarily the print operator simply prints out the comma separated fields you specify, with no trailing newline or record separator assumed. 

Also: $ORS, $OUTPUT_RECORD_SEPARATOR, output_record_separator HANDLE EXPR
 
 
$"
  
Delimiter used when interpreting an array as a scalar. This value separates each element of the array in the resulting string - default a space. 

Also: $LIST_SEPARATOR
 
 
$;
  
The subscript separator for multi-dimensional array emulation, which can be declared directly in v.5. If you refer to a hash element as  

         $foo{$a,$b,$c} 

it really means  

         $foo{join($;, $a, $b, $c)} 

But don't put  

         @foo{$a,$b,$c}      # a slice--note the @ 

which means  

         ($foo{$a},$foo{$b},$foo{$c}) 

Default is "\034", the same as SUBSEP in awk . Note that if your keys contain binary data there might not be any safe value for " $; ". 

Also: $SUBSEP, $SUBSCRIPT_SEPARATOR
 
 
$#
  
The output format for printed numbers. The initial value is '%. 20g'. Deprecated in Perl 5. 

Also: $OFMT
 
 
$%
  
The current page number of the currently selected output channel. 

Also: $FORMAT_PAGE_NUMBER, format_page_number HANDLE EXPR
 
 
$=
  
The current page length (printable lines) of the currently selected output channel. Default is 60. 

Also: $FORMAT_LINES_PER_PAGE, format_lines_per_page HANDLE EXPR
 
 
$-
  
The number of lines left on the page of the currently selected output channel. 

Also: $FORMAT_LINES_LEFT, format_lines_left HANDLE EXPR
 
 
$~
  
The name of the current report format for the currently selected output channel. Default is name of the filehandle. 

Also: $FORMAT_NAME, format_name HANDLE EXPR
 
 
$^
  
The name of the current top-of-page format for the currently selected output channel. Default is name of the filehandle with _TOP appended. 

Also: $FORMAT_TOP_NAME, format_top_name HANDLE EXPR
 
 
$:
  
The current set of characters after which a string may be broken to fill continuation fields (starting with ^) in a format. Default is S<" \n-">, to break on whitespace or hyphens. 

Also: $FORMAT_LINE_BREAK_CHARACTERS, format_line_break_characters HANDLE EXPR
 
 
$^ L
  
What formats output to perform a formfeed. Default is \f. 

Also: $FORMAT_FORMFEED, format_formfeed HANDLE EXPR
 
 
$^ A
  
The current value of the write() accumulator for format() lines. A format contains formline() commands that put their result into $^ A . After calling its format, write() prints out the contents of $^ A and empties. So you never actually see the contents of $^ A unless you call formline() yourself and then look at it. 

Also: $ACCUMULATOR
 
 
$?
  
The status returned by the last pipe close, backtick ( `` ) command, or system() operator. Note that this is the status word returned by the wait() system call, so the exit value of the subprocess is actually ( $? >> 8 ). Thus on many systems, $? & 255 gives which signal, if any, the process died from, and whether there was a core dump. 

Also: $CHILD_ERROR
 
 
$!
  
If used in a numeric context, yields the current value of errno, with all the usual caveats. (This means that you shouldn't depend on the value of " $! " to be anything in particular unless you've gotten a specific error return indicating a system error.) If used in a string context, yields the corresponding system error string. You can assign to " $! " in order to set errno if, for instance, you want " $! " to return the string for error n , or you want to set the exit value for the die() operator. 

Also: $ERRNO, $OS_ERROR
 
 
$@
  
The Perl syntax error message from the last eval() command. If null, the last eval() parsed and executed correctly (although the operations you invoked may have failed in the normal fashion). 

Also: $EVAL_ERROR
 
 
$$
  
The process number of the Perl running this script. 

Also: $PID, $PROCESS_ID
 
 
$<
  
The real uid of this process. 

Also: $UID, $REAL_USER_ID
 
 
$>
  
The effective uid of this process. Example:  

         $< = $>;            # set real to effective uid 

         ($<,$>) = ($>,$<);  # swap real and effective uid 

Also: $EUID, $EFFECTIVE_USER_ID
 
 
$(
  
The real gid of this process. If you are on a machine that supports membership in multiple groups simultaneously, gives a space separated list of groups you are in. The first number is the one returned by getgid() , and the subsequent ones by getgroups() , one of which may be the same as the first number. 

Also: $GID, $REAL_GROUP_ID
 
 
$)
  
The effective gid of this process. If you are on a machine that supports membership in multiple groups simultaneously, gives a space separated list of groups you are in. The first number is the one returned by getegid() , and the subsequent ones by getgroups() , one of which may be the same as the first number. 

Note: " $& lt; ", " $& gt; ", " $( " and " $) " can only be set on machines that support the corresponding set[re][ug] id() routine. " $( " and " $) " can only be swapped on machines supporting setregid() . 

Also: $EGID, $EFFECTIVE_GROUP_ID
 
 
$0
  
Contains the name of the file containing the Perl script being executed. Assigning to " $0 " modifies the argument area that the ps(1) program sees. This is more useful as a way of indicating the current program state than it is for hiding the program you're running. 

Also: $PROGRAM_NAME
 
 
$[
  
The index of the first element in an array, and of the first character in a substring. Default is 0, but you could set it to 1 to make Perl behave more like awk (or Fortran) when subscripting and when evaluating the index() and substr() functions. 

As of Perl 5, assignment to " $[ " is treated as a compiler directive, and cannot influence the behavior of any other file. Its use is discouraged.
 
 
$]
  
The string printed out when you say perl -v . It can be used to determine at the beginning of a script whether the perl interpreter executing the script is in the right range of versions. If used in a numeric context, returns the version + patchlevel / 1000. Example:  

    # see if getc is available 

    ($version,$patchlevel) = 

             $] =~ /(\d+\.\d+).*\nPatch level: (\d+)/; 

    print STDERR "(No filename completion available.)\n" 

             if $version * 1000 + $patchlevel < 2016; 

or, used numerically,  

    warn "No checksumming!\n" if $] < 3.019; 

Also: $PERL_VERSION
 
 
$^ D
  
The current value of the debugging flags. 

Also: $DEBUGGING
 
 
$^ F
  
The maximum system file descriptor, ordinarily 2. System file descriptors are passed to exec() ed processes, while higher file descriptors are not. Also, during an open() , system file descriptors are preserved even if the open() fails. (Ordinary file descriptors are closed before the open() is attempted.) Note that the close-on-exec status of a file descriptor will be decided according to the value of $^ F at the time of the open, not the time of the exec.  

Also: $SYSTEM_FD_MAX
 
 
$^ I
  
The current value of the inplace-edit extension. Use undef to disable inplace editing. 

Also: $INPLACE_EDIT
 
 
$^ P
  
The internal flag that the debugger clears so that it doesn't debug itself. You could conceivable disable debugging yourself by clearing it. 

Also: $PERLDB
 
 
$^ T
  
The time at which the script began running, in seconds since the epoch (beginning of 1970). The values returned by the -M , -A and -C filetests are based on this value. 

Also: $BASETIME
 
 
$^ W
  
The current value of the warning switch, either TRUE or FALSE. 

Also: $WARNING
 
 
$^ X
  
The name that the Perl binary itself was executed as, from C's argv[0] . 

Also: $EXECUTABLE_NAME
 
 
$ARGV
  
The name of the current file when reading from <>.
 
 
@ARGV
  
The array @ARGV contains the command line arguments intended for the script. Note that $# ARGV is the generally number of arguments minus one, since $ARGV [0] is the first argument, NOT the command name. See " $0 " for the command name.
 
 
@INC
  
The array @INC contains the list of places to look for Perl scripts to be evaluated by the do EXPR , require , or use constructs. It initially consists of the arguments to any -I command line switches, followed by the default Perl library, probably "/usr/local/lib/perl", followed by ".", to represent the current directory.
 
 
%INC
  
The hash %INC contains entries for each filename that has been included via do or require . The key is the filename you specified, and the value is the location of the file actually found. The require command uses this array to determine whether a given file has already been included. 
 
 
$ENV {expr}
  
The hash %ENV contains your current environment. Setting a value in ENV changes the environment for child processes.
 
 
$SIG {expr}
  
The hash %SIG is used to set signal handlers for various signals.

Using Environment Variables

These are available as the '%ENV' hash, and retrieved as follows:     $variable = $ENV{'variable'};

The following are commonly used:

CONTENT_LENGTH	The number of bytes of data passed through standard input via the POST method.
DOCUMENT_ROOT	Path on the server to the web site (root URL) being accessed.
GATEWAY_INTERFACE	Protocol used to communicate with the server.
HTTP_COOKIE	The contents of all cookies that are visible to the script - each cookie delimited by a semicolon (';').
HTTP_USER_AGENT	The name (eg. 'Mozilla') and version (eg. '4.03') of the browser used to access the server and the type of platform used by the browser (eg. 'Macintosh;I;PPC').
HTTP_REFERER	The URL of the page that was being viewed when the script was accessed, either via a link or by direct entering o fthe URL. Can be used to check that the user has come from a recognised source, to help with security.
QUERY_STRING	Holds the data input via the GET method.
REQUEST_METHOD	Either "POST" or "GET", depending on what is set on the method attributes of the form tag. NB This value will be "POST" if method="POST", even if some data is passed via GET (eg. data appended directly to the action URL). The following will cause both methods to be used, even though $REQUEST_METHOD="POST":
SERVER_SOFTWARE	Name and version of the web server.

Running a perl program

Explicitly invoking the Perl compiler/interpreter
(note that >>> is prompt only)
>>>perl program.pl
Many command line options are available here eg.
-w print all warnings
-c check but do not execute
-d run under debugger control

Implicitly invoking the Perl compiler/interpreter
It is system specific. e.g. Unix
#!/usr/bin/perl
print "Please enter your name: ";
One can also use -w etc options here also
Eg. #!/usr/bin/perl -w

Other thing is changed the permission of perl file by chmod command to executable and run it as normal executable file.

Command line arg in perl

In perl, command-line arguments are stored in the array named @ARGV.

$ARGV[0] contains the first argument, $ARGV[1] contains the second argument, etc. So if you're just looking for one command line argument you can test for  $ARGV[0], and if you're looking for two you can test for  $ARGV[1], and so on. (Or you can use a loop, as shown below.)
$#ARGV is the subscript of the last element of the @ARGV array, so the number of arguments on the command line is $#ARGV + 1.

Here's a simple Perl program that prints the number of command-line arguments it's given, and the values of the arguments:

$numArgs = $#ARGV + 1;
print "thanks, you gave me $numArgs command-line arguments:\n";

foreach $argnum (0 .. $#ARGV) {

   print "$ARGV[$argnum]\n";

}

When a program is executed, the command line arguments go into a special array called @ARGV.And $_
is the special variable which is used as current element of @ARGV.
Eg.

foreach (@ARGV) {
        print "$_\n";
}

This program prints the command line arguments on the screen.


Another useful way to get parameters into a program -- this time without user input. The relevance to filehandles is as follows. Run the following perl script as:

perl myscript.pl stuff.txt out.txt
while (<>) {
        print;
}

If you don't specify anything in the angle brackets, whatever is in @ARGV is used instead. And after it finishes with the first file, it will carry on with the next and so on. You'll need to remove non-file elements from @ARGV before you use this.

It can be shorter still:

perl myscript.pl stuff.txt out.txt

print while <>;

This takes a little explanation. As you know, many things in Perl, including filehandles, can be evaluated in list or scalar context. The result that is returned depends on the context.
If a filehandle is evaluated in scalar context, it returns the first line of whatever file it is reading from. If it is evaluated in list context, it returns a list, the elements of which are the lines of the files it is reading from.
The print function is a list operator, and therefore evaluates everything it is given in list context. As the filehandle is evaluated in list context, it is given a list !
Who said short is sweet? Not my girlfriend, but that's another story. The shortest scripts are not usually the easiest to understand, and not even always the quickest. Aside from knowing what you want to achieve with the program from a functional point of view, you should also know wheter you are coding for maximum performance, easy maintenance or whatever -- because chances those goals may be to some extent mutually exclu

Avoiding mistakes in perl

You can control perl's level of "strictness" using command line switches and pragmas

the -w switch

#!/usr/local/bin/perl -w

# deliberate_mistake.pl

$variable = 5; $varaible++;

print "new value = $variable\n";

>>>deliberate_mistake.pl
Name "main::varaible" used only once: possible typo at deliberate_mistake.pl line 5.
new value = 5

the strict pragma

This forces all variables to be explicitly declared using the my keyword.

#!/usr/local/bin/perl -w

# deliberate_mistake2.pl

use strict; # forces all variables to be declared

my $variable = 5; $varaible = $variable + 1;

print "new value = $variable\n";

Notice this would not get caught by the -w switch

>>> deliberate_mistake2.pl
Global symbol "$varaible" requires explicit package name at deliberate_mistake2.pl line 7.
Execution of lectures/1_intro/deliberate_mistake2.pl aborted due to compilation errors.